The invention relates to a process and an apparatus for producing polystyrene foam, as well as foam blocks and boards (such as plates and panels) produced therewith.
Of particular relevance in this technical field are GB-A-1,220,053 and GB-A-1,230,992 of Imperial Chemical Industries Limited (ICI). Prior to the date of application of said British patents, foaming tests of synthetic resins using substantially inert inorganic gases, especially CO2, had already been carried out. These tests did not give the desired results, at least not for foam thicknesses of more than a few mm, owing mainly to the low solubility of CO2 in the polymeric mixture or melt to be foamed, which made it necessary to operate at high pressures throughout the process (resulting in higher costs), giving rise to high temperatures, with the result that the temperature of the melt at the time of the final extrusion to the atmosphere (below, xe2x80x9cfinal extrusionxe2x80x9d) is too high to allow the foam features to be controlled properly; moreover, the polymeric mixture may be subject to thermal degradation. As a result, a foam of very high density is obtained, and foam boards of commercial grade can only be achieved for thicknesses in the order of a few mm, since it is not possible to incorporate in the melt the amount of CO2 necessary for obtaining greater thicknesses.
Said documents GB-A-1,220,053 and GB-A-1,230,992 established, toward the end of the 1960s, the general basis for producing specific synthetic resin foams, including those of polystyrene. Particularly, GB-A-1,220,053 describes xe2x80x9ca process for the production of foamed thermoplastic polymers in which the pressure is released from a pressurised homogeneous mixture of the molten thermoplastic polymer and a blowing agent system, said system comprising a first blowing agent which is completely miscible with the molten thermoplastic polymer at the said pressure and which is a solvent for said thermoplastic polymer under the conditions of temperature and pressure of the homogeneous mixture and a second different blowing agent which has a solubility of at least 0.01% by weight in the thermoplastic polymer and a critical temperature less than the temperature of the homogeneous mixture at the point when said pressure is released, said first blowing agent having a boiling point 10xc2x0 C. or more below said temperature of the mixturexe2x80x9d (claim 1). Moreover, it clearly states that the process can be applied to the foaming of xe2x80x9cpolystyrenexe2x80x9d (page 1, col. 2, line 53), that xe2x80x9cthe first and the second blowing agents are selected so that they are inert with respect to the thermoplastic polymer under the pressure and temperature conditions of the pressure vesselxe2x80x9d (page 2, col. 1, lines 40-44), that xe2x80x9cthe most suitable substances for use as the first blowing agent are liquids whose boiling points at atmospheric pressure are greater than room temperaturexe2x80x9d (page 2, col. 2, lines 69-72), that xe2x80x9cethanolxe2x80x9d can be used as the first blowing agent (page 2, col. 2, line 83), that it was found that xe2x80x9ccarbon dioxidexe2x80x9d (page 3, col. 1, line 17) is a substance which can be used as the second blowing agent, that it is preferable to use xe2x80x9cas much of the second gaseous blowing agent (CO2) as possiblexe2x80x9d, but stating that this amount of CO2 is usually less than 10% by weight (page 3, col. 1, lines 29-34), and suggesting that, in fact, the foaming produced by the first blowing agent (for example, ethanol) alone may give rise to a small number of very large bubbles or cells, which is detrimental to the commercial value of the product (page 3, column 1, lines 10-14).
In the years following the filing date of said ICI patents, manufacturers tended more and more towards using halocarbon blowing agents (also foreseen in said British patents), either alone or in various mixtures. However, for environmental reasons, since the 1980s, attempts have been made to decrease the use of halocarbon blowing agents and everything points to the increased use of CO2 as the main blowing agent. From among the patent documents reflecting these trends, the following may be mentioned: WO 93/22371, EP-A-0 318 846, EP-A-0 411 923, EP-A-0 464 581, EP-A-0 597 375, DE 39 43 265 A1, U.S. Pat. No. 5,158,986, U.S. Pat. No. 5,244,927, U.S. Pat. No. 5,250,577 and U.S. Pat. No. 5,269,987, some of them also involving the use of ethanol as xe2x80x9cblowing agentxe2x80x9d.
Apparatuses of the general kind defined in preamble of present claim 7 are know from EP-A-0 528 536 and U.S. Pat. No. 4,436,679.
As may be gathered from said patent documents of ICI, it would be ideal to work with a single blowing agent, namely CO2. However, its low solubility in the polymer and the other drawbacks referred to above make it necessary to use CO2 in conjunction with other different agents which below that of the polymer melt at the moment of the final extrusion will inevitably have, to a greater or lesser extent, a foaming effect, and in that sense such an agent can qualify as a ?blowing agent?, as is done, for example, in said patent documents of ICI when referring to the xe2x80x9cfirst blowing agentxe2x80x9d. However, especially when the agents employed jointly with CO2 are lower alcohols, the foaming effects (inevitably) produced are in general undesirable, owing to the fact that they tend to give irregular bubbles or cells of which large-sized and/or open ones predominate, rather than cells with the desired features produced by using the proper blowing agent, i.e. CO2. For this reason, agents other than CO2 will not be referred to in the present specification as xe2x80x9cblowing agentsxe2x80x9d but as xe2x80x9ccontrol agentsxe2x80x9d since, rather than being used for foaming, they are used for enabling the foaming produced basically by the proper blowing agent (CO2) to proceed adequately.
The present inventors have confirmed through experiments that ethanol acts as a solvent for polystyrene, thereby reducing the viscosity of the melt, making it possible to work at lower pressures along the entire production line, including the final extrusion head, while using the same amount of CO2 incorporated in the melt. A decrease in the viscosity also results in a decrease in both the internal friction of the melt and its friction with the mechanical components it makes contact with and, as a result of this, in a decrease in the temperature of the melt, which is of special importance near the die of the final extrusion head. The present inventors have also confirmed through experiments the beneficial effects of ethanol, by virtue of absorbing heat from the melt when the foaming occurs.
However, the experiments carried out by the present inventors using ethanol as control agent for the foaming, mainly produced by CO2, have also revealed its drawbacks, which chiefly consist of the fact that the foams obtained using ethanol take a considerable amount of time to reach he final value of some of their physical properties which are considered critical, such as their high-temperature dimensional stability, compression strength, and behaviour towards fire or self-extinction capacity, and that (as already stated) ethanol tends to produce foaming with large and open cells.
The aim of the present invention is to overcome these drawbacks.
The inventors have found that where ethanol is used as control agent for the foaming produced by CO2 the best foam features are obtained if the minimum amount of ethanol is added, just sufficient to give to the melt the proper viscosity for absorbing the highest possible amount of CO2, and to keep the foaming produced by ethanol at the minimum possible amount.
Moreover, the inventors have found that this reduction in the amount of ethanol used would by itself have the potential drawback of making it difficult to achieve the required decrease in temperature, since one of the effects of ethanol is to act as coolant of the melt. In order to avoid this drawback, the inventors propose the use of a second control agent consisting of H2O2 which is added to the melt subsequent to the addition of ethanol.
The fact that H2O2 does not dissolve polystyrene and therefore does not modify its features has the effect that the physical properties of the foam are maintained or improved and that the foam can be obtained with greater thicknesses than when using ethanol alone as the control agent for controlling the foaming produced by CO2. The use of a much smaller amount of ethanol allows that said physical properties which are considered critical (dimensional stability, strength and self-extinction capacity) are achieved more rapidly.
Bearing in mind the foregoing, the invention provides, according to a first aspect, a process for producing polystyrene foam, in the initial step of which process a mixture of polystyrene with customary nucleating agents, plasticisers and additives is plastified at a pressure and temperature respectively above atmospheric pressure and room temperature in order to form a melt, which is suddenly decompressed and cooled to atmospheric pressure and room temperature, on being extruded through a final extrusion die, in which process a blowing agent designed to produce the desired foaming and only consisting of CO2 is injected into the melt so that said blowing agent is dissolved in the melt, in which process a first and a second control agent are also injected into the melt, the first control agent being designed to dissolve and cool the melt and consisting of ethanol, while the second control agent is only designed to cool the melt and consists of H2O2, the injected amount of ethanol being such that it is just sufficient for achieving the highest possible dissolution of CO2 in the melt, and the injected amount of H2O2 being such that the foaming produced by ethanol is minimised.
According to an optional feature, the ethanol is injected into the immediate proximity of the injection point of CO2 and the H2O2 is injected downstream from that point, when the injections of the blowing agent and the ethanol have already partially cooled the melt.
According to another optional feature, immediately before the final extrusion, the melt to be extruded comprises 2.25-5% by weight of CO2, 0.3-3.0% by weight of ethanol and 0.2-1.7% by weight of H2O2, the remainder being polystyrene and customary nucleating agents, plasticisers and additives.
According to another optional feature, immediately before the final extrusion, the melt to be extruded comprises 3.0-4.0% by weight of CO2, 0.6-1.25% by weight of ethanol and 0.25-1% by weight of H2O2, the remainder being polystyrene and customary nucleating agents, plasticisers and additives.
According to another optional feature, the temperature and pressure profiles of the process are kept at decreasing temperature values in the range of 200 to 100xc2x0 C. and at decreasing pressure values in the range of to 7.6 MPa (about 200 to 76 bar), respectively, CO2 always being maintained under supercritical conditions until the final extrusion.
According to another optional feature, the polystyrene has a molecular weight of less than about 150,000, a melt flow index of about 20 g/10 min (ISO 1133H) and a VICAT VST B 50 (according to ISO 306 B 50) softening temperature above 100xc2x0 C.
According to another aspect, the invention provides an apparatus for carrying out the above defined process, which comprises a kneader-extruder and a dynamic mixer arranged in series, the first of which comprises means for effecting the initial plastifying of the mixture into a melt and the injection of CO2 and ethanol, and the second of which is provided with a cooling device and feeds the melt into an extrusion head which carries the final extrusion die, there being means for the injection of H2O2, between the kneader-extruder and the dynamic mixer, in which apparatus a first static mixer and a second static mixer are inserted respectively immediately downstream of the kneader-extruder and immediately downstream of the dynamic mixer, and the means for the injection of H2O2 are arranged between the kneader-extruder and the first static mixer.
According to another optional feature, the apparatus comprises, upstream from the kneader-extruder, a first tank for receiving CO2 from an external source, the first tank being maintained under pressure and temperature conditions of the same range as those of the external source, a second tank for CO2 in which this is maintained under a pressure of about 7 MPa (about 70 bar) and at ambient or room temperature, and an injection pump for pumping CO2 from the second tank and injecting it into the kneader-extruder and capable of raising the pressure of CO2 from about 7 MPa (about 70 bar) to about 30 MPa (about 300 bar).
According to another optional feature, the flow of CO2 from the first tank to the second tank is effected by means of another pump, downstream of which a heating device is provided which raises the temperature of the CO2 leaving the another pump to about ambient or room temperature.
According to another optional feature, the flow of CO2 between the second tank and the injection pump passes through a cooling device which lowers the temperature of CO2 to prevent it from being overheated during the pumping operation.
According to another optional feature, the injection pump is a cooled one.
According to another optional feature, the cooling of the injection pump is effected by cooling the pump head.
According to another optional feature, the process is carried out in an apparatus such as the one defined in the immediately preceding paragraphs and comprises the steps of:
a) plastifying into a melt a mixture of polystyrene with customary nucleating agents, plasticisers and additives in the kneader-extruder, the polystyrene having a molecular weight of less than about 150,000, a melt flow index of about 20 g/10 min (ISO 1133H) and a VICAT VST B 50 (according to ISO 306 B 50) softening temperature above 100xc2x0 C.;
b) injecting into the melt, in the kneader-extruder, CO2 on the one hand and ethanol on the other hand, at a pressure of about 20 MPa (about 200 bar);
c) transferring the melt from the kneader-extruder to the first static mixer and injecting H2O2 into the melt between the kneader-extruder and the first static mixer;
d) homogenising the melt in the first static mixer;
e) transferring the melt from the first static mixer to the dynamic mixer in which homogenisation is accompanied by a decrease in the temperature and the pressure to which the melt is subjected;
f) transferring the melt from the dynamic mixer to the second static mixer, in which the decrease in the temperature and the pressure to which the melt is subjected continues;
g) transferring the melt from the second static mixer to the final extrusion head, in which the temperature and pressure are controlled so that they remain close to about 100xc2x0 C. and 7.6 MPa (about 76 bar), respectively, the CO2 always being maintained under supercritical conditions until the final extrusion;
the operative conditions being such
that at the time of the final extrusion, the melt comprises 2.25-5% by weight of CO2, 0.3-3.0% by weight of ethanol and 0.2-1.7% by weight of H2O2, the remainder being polystyrene and customary nucleating agents, plasticisers and additives,
that the blocks or boards produced, 42 days after their production and after having been subjected to heating at 70xc2x0 C. for 2 days, have undergone a decrease in each of their linear dimensions (length, width and thickness) of less than 5% with regard to the original dimension, and
that the test for determining the self-extinction coefficient, carried out on said blocks and boards one hour after they have been extruded, gives a flame height of less than 11 cm.
According to another aspect, the invention provides foam blocks and boards produced using the above mentioned process or apparatus, characterized in that 42 days after their production and after having been subjected to heating at 70xc2x0 C. for 2 days they have undergone a decrease in each of their linear dimensions (length, width and thickness) of less than 5%, with regard to the original dimension.
According to another optional feature of the blocks and boards according to the invention, the test for determining the self-extinction coefficient, carried out on said blocks and boards one hour after they have been extruded, gives a flame height of less than 11 cm.